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Another question for you evolution buffs

mslxl

Thinker
M
Joined
Mar 5, 2006
Messages
233
I see trees on very steep slopes that emerge perpendicular to the slope, then, very quickly, turn upward to grow vertical. Is this currently explained as:
-- some sort of genetic programming, with details still to be understood?
or
-- some kind of natural selection within the organism, by which cells grow in random directions and the ones that grow upward are ultimately rewarded through exposure to sunlight?

Thank you.

mslxl
 
The movement of many chemicals in plants is governed by gravity. A growth hormone that either causes cells to be larger or to replicate faster gathers in the lower part of the plant, so one side grows longer than the other, causing it to bend upwards. Once the plant is vertical all sides have the same concentration and it grows straight up. I can't remember exact details, but that's the basic mechanism.
 
There is also a second mechanism responsible for this effect - the one which causes all plants to grow towards the light source. This works because of a chemical which inhibits growth where there is the greatest sunlight, causing relatively faster growth on the darker side, and tipping the plant towards the light - or in this case causing the shoot to turn vertical, because of the lack of light on the underside of the horizontal growth. If the light levels are very low on both sides then the shoot will grow fast on both sides, in a desperate bid to reach bright light before it runs out of steam.
 
Having no special knowledge about this, I would speculate that all trees are genetically programmed to grow toward the sun. We just don't think about it because, in most cases, that's straight up, the way trees are "supposed" to grow.
 
A third cause can be soil creep. local soil slippage, landslides etc. These can tilt a growing tree away from the vertical. If it survives, it may resume growth towards the vertical, causing a bend in the tree. It's occasionally possible to map the extent of landslips in this way, where a whole area of trees have shifted uniformly and survived.
 
JamesDillon said:
Having no special knowledge about this, I would speculate that all trees are genetically programmed to grow toward the sun. We just don't think about it because, in most cases, that's straight up, the way trees are "supposed" to grow.
Click to expand...

Yes. The only plants which do not necessarily grow towards the sun are climbers and creepers, which respond to contact with other plants, as demonstrated by the following time-lapse clip of a bramble "feeling" it's way across the forest floor at about ten times the growth rate of anything else in the shot:

http://www.bbcmotiongallery.com/Cus...d=95a4adc3-531b-4891-9a37-018af40d41ee&page=8
 
Soapy Sam said:
A third cause can be soil creep. local soil slippage, landslides etc. These can tilt a growing tree away from the vertical. If it survives, it may resume growth towards the vertical, causing a bend in the tree. It's occasionally possible to map the extent of landslips in this way, where a whole area of trees have shifted uniformly and survived.
Click to expand...


Ditto.
 
Darwin himself did a lot of experiments into plant motion. He reasoned that climbers and creepers were spread so diversely among plant genera that it would be extraordinary if the faculty of motion had evolved separately each time, and that therefore all plants must be capable of motion; which he demonstrated to be true for a large sample of genera.
 
Phototropism! Alec, I want points for this!!! However I do not know Jack Squat about how it works, all I know is that it was demonstrated in my HS biology class and readily apparent to all gardeners. Why does pruning "work"??? It just seems to stand to reason. But there were things done on space missions, as I remember, that were designed to test out whether these plants are looking for photons or following gravity or both or neither. I think it came out that phototropism is right. Somebody help me here.
 
BPScooter said:
Phototropism! Alec, I want points for this!!! However I do not know Jack Squat about how it works, all I know is that it was demonstrated in my HS biology class and readily apparent to all gardeners. Why does pruning "work"??? It just seems to stand to reason. But there were things done on space missions, as I remember, that were designed to test out whether these plants are looking for photons or following gravity or both or neither. I think it came out that phototropism is right. Somebody help me here.
Click to expand...

It's definately both. Put some onions or potatoes in a completely dark cupboard and leave them for a few weeks. The ones that sprout always point straight up, even if the sprouts come from the bottom or side. Now open the door so they have direct sunlight, they will bend over to point towards it. If you turn them they will bend to point up (or towards the light if they have one).
 
BPScooter said:
Why does pruning "work"???
Click to expand...

That is a bizarre question. Do you mean to ask "why does the plant start sprouting from the bottom if you cut the top off?" If you are, then the answer is that there is a growth-inhibiting hormone produced just beneath each growing shoot which descends through the plant and inhibits the growth of the vegetative buds lower down. If you cut off the top, the supply of inhibitor is also cut off and the lower buds start sprouting.
 
Cuddles said:
It's definately both. Put some onions or potatoes in a completely dark cupboard and leave them for a few weeks. The ones that sprout always point straight up, even if the sprouts come from the bottom or side. Now open the door so they have direct sunlight, they will bend over to point towards it. If you turn them they will bend to point up (or towards the light if they have one).
Click to expand...

Have you checked to see if an equal number grow straight down?:eye-poppi
 
UndercoverElephant said:
That is a bizarre question. Do you mean to ask "why does the plant start sprouting from the bottom if you cut the top off?" If you are, then the answer is that there is a growth-inhibiting hormone produced just beneath each growing shoot which descends through the plant and inhibits the growth of the vegetative buds lower down. If you cut off the top, the supply of inhibitor is also cut off and the lower buds start sprouting.
Click to expand...

I thought that it was also because the auxins then could no longer concentrate in the growing cells of the main shoot and that the redistribution of the auxins also caused the growing cells to accelerate in other areas of the plant. Which is a positive growth cycle, but I could be very wrong.
 
I’d like to make a general point to mslxl. If you’re trying to understand evolution by plugging away at case studies which have you stumped, you’re going to generate a lot of very interesting threads which nonetheless teach you little.

Evolution is an elaborate edifice -- if you want to understand it, there’s no substitute to putting your nose to the grindstone and sweating it out with a textbook or two. Local libraries often carry college 101 level books.

An alternative is to read a bucket load of Dawkins, Gould, et. al. You’ll get an entertaining grounding, but they may still leave you picking at case studies. In fact, this is the mistake anti-evolutionists often make. They think that finding a few cases which they themselves can’t explain in their limited, distorted understanding of evolution somehow make decisive counter-arguments.

Audit a college course. Labor over a textbook. Evolution is not like law -- case studies aren’t ends in themselves.

(Not that I’m discouraging you from starting these threads; I just think they aren’t going to get you very far.)
 
Ryan O'Dine said:
Evolution is an elaborate edifice -- if you want to understand it, there’s no substitute to putting your nose to the grindstone and sweating it out with a textbook or two. Local libraries often carry college 101 level books.
Click to expand...

....or get hold of a copy of the best TV series ever made on the subject, Attenborough's "Life on Earth":

http://www.amazon.co.uk/exec/obidos/ASIN/B00004CJF3

If you still don't believe/understand it after watching this series, there is no hope for you.
 
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